1. Field of the Invention
The present invention relates to an electrically heated window, and more particularly to a window heated by the passage of current through an electrically resistant area heating means included in the window. The heating means may comprise fine, closely spaced wires, or an electrically conducting film. In electrically heated windows, busbars may be printed in conductive ink, or formed from solid conductive material, e.g. metal strip. The present invention is concerned with windows having the latter type of busbar.
2. Description of the Related Art
Such a window is normally laminated, comprising at least one ply of interlayer material (the "interlayer") sandwiched between at least two plies of transparent glazing material, for example glass or plastics material. A window of this type may be used to disperse condensation or ice, and may be installed in a vehicle; for example as the windscreen or backlight of a car, or the windscreen of a locomotive or an aircraft.
The electrical connections to the heating means each include at least one, and normally two or four busbars comprising a strip of electrically conductive material. Especially in vehicles with a nominal 12V electrical system, such a busbar carries a considerable current, possibly 25 to 30 A, and so it is desirable that the busbar is of adequate cross-sectional area to avoid occurrence of a significant voltage drop and production of heat in the busbar. The thickness of the busbar is limited by the fact that it is normally included between two of the plies laminated together; hence adequate current-carrying capacity is provided by specifying a sufficient width for the busbar, which normally is from at least 3 mm wide up to 6 mm wide.
In vehicles, and especially but not exclusively in cars, the external aesthetic appearance of the vehicle is considered important, and so it is considered desirable to obscure the busbar from external view. This is achieved by positioning the busbar as close as possible to the edge of the window, and then concealing it by obscuration means. Such means may take the form of a finishing trim strip which overlaps the edge of the window and is applied by the vehicle manufacturer, or a ceramic ink or organic primer (normally black) applied in a peripheral band to an inner (i.e. facing towards the vehicle interior) face of the outermost ply of glazing material.
In either case, the busbar would become visible if it were positioned sufficiently far inboard from the peripheral edge of the window, and so vehicle manufacturers normally specify a strict limit on the maximum distance from the edge of the window that a busbar may be positioned (hereinafter referred to as the "manufacturers' limit"). It is relatively straightforward to position accurately a straight length of busbar in proximity to a straight edge of a window; however difficulties may be experienced at corners, for it is desirable, to comply with the manufacturers' limit, that the busbar turns the corner without diverging from the peripheral edge of the window, i.e. remains substantially parallel to the edge. When the shape of the corner of the window approximates to two straight lines meeting at a particular angle, e.g. a right angle, it is known to fold the strip forming the busbar at a corresponding angle also, and the busbar thus formed may be positioned in the corner while complying with the manufacturers' limit specified on distance from the peripheral edge of the window. A busbar assembly in which the busbar is "doubled" upon itself to turn a right angle is disclosed in U.S. Pat. No. 3,612,745.
It is more difficult to comply with the manufacturers' limit by folding the busbar strip when the corner of the window is radiused, and it becomes increasingly difficult with larger radii of curvature. For example, the corner may have the shape of an arc of a circle, or, as is more frequently the case, the corner may have a curvature of compound radius, i.e. the radius varies around the corner. It is not possible to position the busbar in such a corner within the specified limit if the busbar is formed by making a single fold in the busbar strip; two or more folds are required. These folds are of course no longer each at the angle turned by the corner; when the folds are made, the angle of each fold must be estimated so that the sum of the angles corresponds to the total angle turned by the corner of the window. The total angle is usually not an easily estimated angle, such as a right angle, given that a typical vehicle window is trapezoidal rather than rectangular. An example of a corner configured according to the prior art is shown in FIG. 3. This method of causing the busbar to turn a corner requires skilled manual labour, and does not lend itself to automation. It has also been observed that it gives rise to an increased level of rejection. All these factors increase the cost of making the product.
On investigation, it has been found that the increased rejection relates to inaccuracies in estimating the position and angle of the folds in the busbar strip. From the information given so far, it will be evident that rejection will occur if any part of the busbar is not sufficiently close to the peripheral edge of the window, i.e. if the limit specified by the vehicle manufacturer is not adhered to, and the corners of the window are instances where the limit may easily be exceeded.
As a result, operators attempt to position the folded busbar as far into the corner of the window as possible, but this practice has itself in the past given rise to additional rejection. Rejection in these instances has been due to poor electrical continuity, and it has now been realised that this rejection constitutes a further problem.
The manufacture of a window laminated with an interlayer material such as polyvinylbutyral ("pvb") involves heating the assembly of plies under pressure, e.g. in an autoclave ("autoclaving"), to remove residual air from the assembly, and soften the interlayer so that the plies (and any other parts in contact with the interlayer, e.g. busbars) are adhered together. A side effect of the pressure is that the ply of interlayer material increases in area slightly, and the excess interlayer material protrudes beyond the plies of glazing material. Part of the finishing procedure is to trim the excess interlayer material in a known manner, e.g. by cutting with a sharp blade, or by abrading it with an abrasive belt.
When an operator seeks to position the folded busbar as far into the corner of the window as possible, there is a risk of positioning the folded busbar further into the corner than is actually desirable, e.g. positioning it so that part of the busbar protrudes from between the plies. Alternatively, the folded busbar may be positioned so close to the corner that the increase in area of the interlayer and consequent movement thereof causes part of the busbar to protrude from between the plies along with the interlayer.
In either of these situations, subsequent trimming of the protruding interlayer will normally also result in removal of part of the busbar. This will cause a local reduction in current-carrying capacity of the busbar, possibly leading to a voltage drop and localised overheating of the busbar. Additionally, the part of the folded busbar which is most likely to be removed is the line of the fold itself. When this occurs, there is no physical connection between the folded and unfolded parts of the busbar, consequently electrical continuity becomes dependent on good electrical contact between the two parts. While such good electrical contact may exist when the window is new, the effect of general weathering in service and in particular moisture ingress by capillary action between the two pieces of busbar causes corrosion of the surfaces of the busbar resulting in deterioration of the electrical contact.
It is known to use a thin, flexible expanded mesh of copper foil for a busbar. U.S. Pat. No. 4,323,726 points out the ability of such a material to form small radius bends in its own plane. However, the material is still bent by an operator at the time the busbar is laid down, and so the accuracy of the bend is still subject to operator error. Furthermore, such a mesh has a substantially reduced current-carrying capacity compared with solid strip, and so it must be used in conjunction with a further conductive material such as the conductive paste described in U.S. Pat. No. 4,323,726. This greatly complicates busbar formation, and the paste also brings problems of its own.